Linear variable differential transformer with digital electronics

ABSTRACT

Techniques for coupling with devices that convert displacements into differential voltages and improve the sensitivity of such devices. The disclosed system improves the accuracy and resolution of a transducers such as an LVDT by converting certain parts of the circuit to a digital circuit. One embodiment uses a processor, although other digital processing circuitry may also be used.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional ApplicationSer. No. 60/383,384, filed on May 24, 2002.

BACKGROUND

[0002] A linear variable differential transformer (LVDT) is a positionsensor that can convert mechanical displacements into differentialvoltages. An LVDT conventionally uses a moving part that is moved withina magnetic field created by another part. An output is produced as thepositions of the elements change relative to one another.

[0003] One kind of LVDT is described in applicant's co pendingapplication Ser. No. 10/016475, entitled Improved Linear VariableDifferential Transformer For High Position Measurements. However, thepresent system can be used with any type of transducer which operatesdifferentially, including any LVDT, or any other differentialtransducer.

[0004] The accuracy of the electronics used to process the signal cangreatly affect the output value which is received from the LVDT. Moreprecise electronics will improve the resolution and accuracy of theoutput value.

SUMMARY

[0005] The present system teaches a differential transducer, andimproved electronics which can be used for excitation and signalconditioning in the differential transducer. In an embodiment, thedifferential transducer is an LVDT, which is measuring the movementcreated by an object.

[0006] The system described herein may use digital electronics as theexcitation and signal conditioning electronics and a transducer of thetype disclosed herein.

[0007] In a specific embodiment, the transducer is driven by a phaseshift circuit which periodically inverts phase, and a switching element,which switches a differential output in synchronism with the changing ofthe phase. Both the phase shift circuit, and the switching element areformed by a digital processing element, e.g., a processor. In anotherembodiment, a digital square wave oscillator is formed by amicroprocessor which digitally generates primary and reference waveformsfor the transducer. This may substantially increase the flexibility andsensitivity of the transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other aspects will now be described in detail withreference to the accompanying drawings, in which:

[0009]FIG. 1 shows a first embodiment which produces first and secondout of phase square waves using a digital processor, which square wavesare used for driving excitation and reading of the waveforms;

[0010]FIG. 2 shows a flowchart of operation of the processor of thefirst embodiment;

[0011]FIG. 3 shows a second embodiment in which a digital processor isused to generate a reference wave and to digitally phase shift theoutput; and

[0012]FIG. 4 shows a flowchart of operation of the processor of FIG. 3.

DETAILED DESCRIPTION

[0013]FIG. 1 shows an embodiment. A differential transducer, here anLVDT, produces an output signal having a magnitude related to an amountof movement in a linear direction. The transducer is formed to exploitchanges of inductances between a moving primary and two secondaries,changing as a function of position between these. The output of thisdevice is a signal which is proportional to the position of the movingprimary coil 15. This LVDT may be used for converting motion intovoltage (or voltage into motion) and as such may be used as a number ofdifferent applications. Preferably, however, the transducer is used forhigh precision applications. For example, this may be used to carry outprecision force measurements, for example, by measuring the deflectionof a flexible cantilever with a sharp tip as it pushes or pulls on asurface. The transducer may be used in making force measurements, again,for example, using a cantilever such as a silicon cantilever. The systemmay also be used for surface profiling, in which case a sharp tipattached to a moving stylus is pivoted around a jewel. Anotherapplication may use the system as part of a profilometer, both as asensor and otherwise. Yet another application may be as part of anatomic force microscope, such as described in applicant's co-pendingapplication or in U.S. Pat. No. RE34,489. The system may also be used aspart of a molecular force probe. This system may be ideal for devicesthat convert very small mechanical displacements, for example as smallas subnanometer level (<1 mm), into these differential voltages. Whilethe embodiment describes a linear variable differential transformer orLVDT, this system may also be used with the capacitive-based sensor. Inaddition to the applications described above, this system may also beused in molecular force measurements, manipulation technology,lithographic manufacturing, nanometer scale surface profiling, and inmany different aspects of nanotechnology.

[0014] In the embodiment of FIG. 1, a digitally controlledmicroprocessor 80 produces a square wave output 81 based on storedinstructions, which will control the periodic phase inversion in thetransducer. The instructions may be stored in a memory, or may beembedded within the processor itself. The square wave output is filteredby filter 24 in order to produce a sine wave 23. The filter 24 may be alow pass filter that effectively removes all harmonics of the squarewave above the fundamental. Moreover, the filter is optimized forstability with respect to variations in temperature. Hence, the sinewave which is produced may be substantially pure. The sine wave 23 isamplified and/or buffered by current buffer 25 to produce sine wave 77.The output sine wave 77 is applied to the primary 15 of the transducer.

[0015] The sine wave 77 which is applied to the primary may be asubstantially perfect frequency and amplitude and virtually noise free.Any defects may be extremely important since any noise or frequency oramplitude instability in the drive can appear in the demodulated outputsignal. In the embodiment shown in FIG. 1, the primary moves relative tothe secondary, however it should be understood that the primary can bestationary with the secondary instead moving.

[0016] The movement of the primary induces an induced current into thesecondary 3, 4 which is amplified by the differential amplifier 6 andoutput. The differential amplifier may be a low noise or differentialamplifier which is adapted for coupling to a low impedance input sourcesuch as a coil.

[0017] The output of the differential amplifier is connected to a bufferamplifier 31 and to an inverting buffer amplifier 32. An analog switch33 selects one of the two outputs respectively from the buffer amplifier31 or 32. The analog switch is controlled out of phase with the inputdrive to the primary of the LVDT. In this way, the output signal isselected synchronously with the phase reversal to the primary input.

[0018] The output of buffer amplifier 31 which is fed into the normallyclosed input of an analog switch 33. The output of inverting bufferamplifier 32 is coupled to the normally open input of the switch 33. Theanalog switch is controlled by an inversion waveform, which may be asquare wave which is also produced by the microprocessor 80. This squarewave may be shifted by any desired amount relative to the phase reversalsquare wave 81, by appropriate programming of the microprocessor 80.Moreover, the arrangement of elements 31 and 32 and 33 may be reversedso long as the two parts of this switch are set such that one is openwhile the other is closed.

[0019] Both the square wave driving the primary 15, and also the squarewave driving the analog switch 33, are controlled by the processor. Inthis way, the system uses a single microprocessor to generate an inputphase inversion signal for the differential transducer and also togenerate an output phase inversion operation for the same differentialtransducer. The two square waves can be shifted relative to one another.Either the output square wave 82 driving the analog switch can beshifted relative to the primary square wave 81, or vice versa; all thatmatters is that the relative phase of the primary drive in the referenceare adjustable relative to one another.

[0020] An important feature of the present system is based on theinventor's recognition that a microprocessor has the capacity togenerate a substantially pure and precisely shifted square wave. Thesquare waves may be otherwise identical other than their phase. This maysubstantially increase the flexibility in sensitivity of electronics asdisclosed. Moreover, this may result in a smaller parts count, since thesame processor creates two different waveforms.

[0021] In one embodiment, the opening and closing of the two parts ofswitch 33 may occur 90° out of phase relative to the output signal fromthe amplifier 6.

[0022] The output of the analog switch 33 is fed to a stable low noiselow pass filter 34 that produces a signal that is proportional to theposition of the moving primary coil 15.

[0023] The microprocessor 80 may be any kind of processor including amicrocontroller, digital signal processor, reconfigurable logic, or anyother type of controllable processing device. The processor 80 may becontrolled according to the flowchart of FIG. 2.

[0024] At 200, the system operates to create a first square wave. Thisis done by changing the output logic level from low to high at 205. Inthis way, the processor produces an output transition forming the firstpart of the square wave. The processor then waits, during which theduration of the pulse is formed, at 208. The logic level remains highduring the waiting. At 210, the end of the square wave is signaled, bychanging the output level from high to low. This completes the formationof the first square wave.

[0025] A second square wave is created after a phase shift Φ. The systemwaits for a time Φ at 215, and then proceeds to create another squarewave using the same techniques as described above.

[0026] A second embodiment is shown in FIG. 3. This embodiment uses asimilar basic layout to the system shown in FIG. 1, however operatesusing a digital phase shift.

[0027] The FIG. 3 embodiment uses the processor 800 to create thedigitally created square wave, as in the first embodiment. In addition,however, the output of the differential amplifier is coupled to an A/Dconverter 81. The digitally-converted signal is fed back to theprocessor 800. The processor operates to digitally invert the outputfrom the differential amplifier according to a phase-shifted version ofthe digitally created square wave. That is, in this embodiment, theprocessor 800 carries out the functionality of the analog switch in thefirst embodiment. This may even further decrease the part count. Also,as in the first embodiment, the system uses a single microprocessor togenerate an input phase inversion signal for the differential transducerand also to generate an output phase inversion operation for the samedifferential transducer.

[0028] That is, the digital output 82 from the A/D converter isdigitally processed by the processor 800. The processor 800 carries outthe flowchart shown in FIG. 4.

[0029] At 400, the processor creates the square wave 801 which isapplied to the low pass filter 24 and used in an analogous way relativeto the first embodiment shown in FIG. 1. The processor 800 also receivesbits from the A/D converter 81 at 402. The sense of these bits isselectively inverted at 404, in a sense that is Φ degrees out of phasewith the square wave that was produced at 400. In this way, the bits areinverted in a specified sense relative to the digitally created squarewave. By using a controllable processor, further accuracy in the wavemay be produced, and additional advantages may be obtained. For example,the processor may be used for other functions in the circuit.

[0030] Although only a few embodiments have been disclosed in detailabove, other modifications are possible. For example, while theembodiment extensively discloses use with an LVDT, this system may beused in other similar transducers which use periodic phase inversion.Also, other digital processing elements may be used. All suchmodifications are intended to be encompassed within the followingclaims, in which:

What is claimed is:
 1. A system comprising: a transducer which operatesbased on periodic phase inversion; a programmable processor, whichoperates according to a stored program, said stored program causing saidprocessor to produce a first waveform based on stored instructions whichdrive said transducer.
 2. A system as in claim 1, wherein said firstwaveform is a square wave produced by a first instruction to produce arising edge of the square wave, and a second instruction to produce afalling edge of the square wave.
 3. A system as in claim 1, wherein saidprocessor is also programmable to produce an inversion waveform thatselectively inverts an output of the transducer, at a timing having aspecified phase relationship with said first waveform.
 4. A system as inclaim 3, wherein further comprising an analog switch, coupled to receivean output of said transducer, and wherein said inversion waveformcontrols said analog switch.
 5. A system as in claim 3, furthercomprising signals defining a first noninverting output of saiddifferential amplifier, and a second inverted output of saiddifferential amplifier, and an analog switch, having two inputsrespectively receiving said inverting output and said non-invertedoutput, and wherein said inversion waveform controls said analog switch.6. A system as in claim 1, wherein said processor also receives anoutput of said transducer, and includes a program which causes saidprocessor to selectively invert said output of said transducer at aspecified timing.
 7. A system as in claim 6, wherein said specifiedtiming is at a specified phase shifted relative to said first waveform.8. A system as in claim 1, further comprising an A/D converter, coupledto receive an output of said transducer, and to produce a digital outputindicative of an output of said transducer.
 9. A system as in claim 8,wherein said processor receives said digital output indicative of saidoutput of said transducer, and said stored program is also operative toselectively invert a sense of bits within said digital output, at aspecified timing relative to said first waveform.
 10. A system as inclaim 1, wherein said first waveform is a square wave, and furthercomprising a low pass filter which filters all but a fundamentalfrequency of said square wave to produce said first waveform.
 11. Asystem as in claim 10, wherein said transducer is a linear variabledifferential transformer.
 12. A system as in claim 11, furthercomprising a cantilever element, coupled to said linear variabledifferential transformer, such that said linear variable differentialtransformer is moved by movements of said cantilever.
 13. A system as inclaim 10, wherein said transducer is a transducer which exploits changeof inductances between a primary and two secondaries.
 14. A system as inclaim 13, wherein said transducer is a transducer capable of analyzingmovements which are less than 1 nm.
 15. A transducer system, comprising:a transducer input part, which operates based on changes of inductancesbetween primary and secondary to produce a differential signal; and asingle structure, which produces both a first phase inversion signal fordriving the transducer input, and which produces an inversion operationwhich selectively inverts an output of the transducer at a specifiedtiming having a specified phase relationship with said first phaseinversion signal.
 16. A system as in claim 15, wherein said singlestructure is formed from a processor.
 17. A system as in claim 15,wherein said single structure is formed from a programmed processorwhich operates according to stored instructions.
 18. A system as inclaim 16, wherein said processor produces a square wave output as saidphase inversion signal, and further comprising a filter which filterssaid square wave output to produce a substantially single frequencysignal.
 19. A system as in claim 18, further comprising a currentbuffer, between said substantially single frequency signal, and an inputto said transducer.
 20. A system as in claim 16, further comprising aninverting structure, coupled to an output of said transducer, andwherein said inversion operation comprises controlling an output to becoupled to either the output of said transducer or the output of saidinverting structure.
 21. A system as in claim 20, further comprising ananalog switch, and wherein said inversion operation comprises a waveformwhich controls said analog switch.
 22. A system as in claim 21, whereinsaid waveform which controls said analog switch is a square wave havinga specified phase relationship with said first phase inversion signal.23. A system as in claim 22, wherein said specified phase relationshipis substantially 90° out of phase.
 24. A system as in claim 16, whereinsaid inversion operation comprises a digital inversion of specifiedparts of the output of the transducer.
 25. A system as in claim 16,further comprising an A/D converter, creating a digital bitstreamindicative of the output of said transducer, and providing said digitalbitstream to said processor, and wherein said processor's selectivelyinverts a sense of said bitstream at said specified phase relationship.26. A system as in claim 25, wherein said specified phase relationshipis substantially 90° out of phase.
 27. A system as in claim 15, furthercomprising a linear variable differential transformer including saidtransducer input part.
 28. A method, comprising: producing a phaseinversion signal for a differential transducer using a digitallycontrollable processor; and also using said digitally controllableprocessor to produce a selected sense inversion of an output of thedifferential transducer.
 29. A method as in claim 28, wherein saidproducing said phase inversion signal comprises producing a square waveusing said processor.
 30. A method as in claim 29, further comprisingfiltering said square wave to produce a substantially pure sine wave,and using said substantially pure sine wave to drive said differentialtransducer input.
 31. A method as in claim 28, wherein said using saiddigitally controllable processor to create said selected sense inversioncomprises producing an output signal having a specified phaserelationship with said phase inversion signal.
 32. A method as in claim31, wherein said specified phase relationship is substantially 90°. 33.A method as in claim 28, wherein said using said digitally controllableprocessor to create said selected sense inversion comprises digitallyinverting specified portions of the output of the differentialtransducer.
 34. A method as in claim 33, further comprising analog todigital converting the output of the differential transducer, andapplying the digital version of the output of the digital transducer tothe digitally controllable processor.
 35. A method as in claim 34,further comprising using the digitally controllable processor isoperable to selectively inverts a sense of the digital signals.
 36. Amethod as in claim 35, wherein said using the digitally controllableprocessor to selectively inverts a sense comprises operating at aspecified phase relationship having a specifies phase relationship withsaid phase inversion signal.
 37. A method as in claim 28, furthercomprising using said differential transducer to convert motion intovoltage.
 38. A method as in claim 37 wherein said motion can be resolvedto a resolution of at least 1 nm.
 39. A method as in claim 37, whereinsaid motion is motion of a cantilever which measures characteristics ofa surface.
 40. A method as in claim 37, wherein said motion is part ofan atomic force microscope.
 41. A method, comprising: using a singleprocessor to generate an input phase inversion signal both for adifferential transducer and to generate an output phase inversionoperation for the same differential transducer.
 42. A method as in claim41, wherein said output phase inversion comprises creating a signal todrive the phase inversion.
 43. A method as in claim 41, wherein theoutput phase inversion comprises selectively inverting a sense of anoutput signal from the differential transducer.